It's interesting to me to see three votes for Dan Robinson. I do know that he teaches. I do know that he doesn't mind having students, but don't use the terms "Sensei" or "Master" in his presence!
If you just come by and say "I want some lessons", he will just have you hang around with him while he goes about what he does, talking all the time. I don't know his exact rates these days, but that sort of teaching could get very expensive very quickly, and you might be frustrated by his teaching style, which is not classically didactic at all. But the impact can be profound, even in a short time, even if you wouldn't think so at the time. Vic or Eric might know his current rates for that sort of teaching. You'll get the most out of it if you are not a raw beginner.
You could just come by on a weekend and find him in the garden. He is constantly striking up conversations with visitors as he goes about his business. If you already know some basics, and just listen and watch and ask a few reasonable questions, you could learn an enormous amount in just a few minutes, then follow it up with a tour of the 250 trees in the garden to drive some of the points home and get some further inspiration, and "chat" with him some more as he works on a tree. Price of admission to the garden is $8.50 I think. I have a friend who spent twenty minutes stopping by, introducing himself, helping him unload a tree from the bed of his pickup out in the parking lot, and talking about the tree while they did so. The impact on his enthusiasm and direction in bonsai has been profound.
Vic and Eric have been fortunate enough to have been accepted as unofficial apprentices for several years now. I don't think Dan either pays or charges them, but they work and learn there all the time in an immersion situation. There is another fellow who does this as well, but less often I think. I think that is the best way, but he will not accept you into this sort of arrangement unless he thinks you are really ready and willing. If he gets a hint that you are just wasting his time, you're out.
Vic and Eric can say more about this whole thing. Hopefully they will chime in.
Hope that helps.